Activation of AMP-activated protein kinase may not be involved in AICAR- and metformin-mediated meiotic arrest in bovine denuded and cumulus-enclosed oocytes in vitro

The adenosine monophosphate-activated protein kinase (AMPK) activators, 5'-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR) and metformin (MET), inhibit resumption of meiosis in bovine cumulus-enclosed oocytes (CEO) and denuded oocytes (DO). The objectives of this study were to: (1) examine the effects of AMPK inhibitors on bovine oocyte meiosis in vitro; and (2) determine if AICAR or MET activates oocyte and/or cumulus cell AMPK. The AMPK inhibitor compound C (CC; 0.5, 1, 5, and 10 mu M) did not reverse the inhibitory effects of AICAR (1mM) and MET (2mM) on bovine oocyte meiosis. Additionally, CC (5 and 10 mu M) inhibited meiosis (p < 0.05) in CEO and DO cultured for 7 h. Okadaic acid (1 mu M) reversed the inhibitory effect of MET (2mM) and CC (5 mu M; p < 0.05) but not of AICAR (1mM). Phosphorylation of the alpha subunit of AMPK on Thr172 is required for activation. Based on western blot analysis, AICAR, MET and CC did not affect Thr172 phosphorylation levels in DO and oocytes from complexes (p > 0.05). In cumulus cells, Thr172 phosphorylation decreased after 3 h of culture (p < 0.05), regardless of the presence of AMPK modulators in the culture medium. Higher concentrations of AICAR (2mM) and MET (10mM) did not affect Thr172 phosphorylation, but phosphorylation on Ser79 of ACC, a substrate of AMPK, was increased in response to MET (p < 0.05). In conclusion, we inferred that the inhibitory effect of AICAR and MET on bovine oocyte meiosis was probably not mediated through activation of AMPK. Moreover, these compounds probably inhibited meiosis through different pathways.